UNIVERSITY OF CALIFORNIA
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
IRRIGATION EXPERIMENTS
WITH PEARS AND APPLES
A. H. HENDRICKSON and F. J. VEIHMEYER
BULLETIN 667
May, 1942
UNIVERSITY OF CALIFORNIA
BERKELEY, CALIFORNIA
CONTENTS
PAGE
Introduction 3
Methods 5
Eesults with pears 7
Nutting orchard 7
Benson orchard 11
Cleland orchard 16
University Farm orchard 23
Prost orchard 27
Observations on maturit}' of pear fruits under dry soil conditions in other
districts 29
Eesults with apples 30
Hotle orchard 30
Jerenich orchard 32
Eowe orchard 33
Discussion of results 38
Conclusions 41
Literature cited 42
IRRIGATION EXPERIMENTS WITH PEARS
AND APPLES1
A. H. HENDRICKSON2 and F. J. VEIHMEYER;!
INTRODUCTION
Pears in California are grown under a wide range of climatic condi-
tions, varying from the comparative mildness of the central-coast region
to the warmth of the interior valleys and foothills. Apples, for the most
part, are grown in the central-coast region, with scattered plantings in
many other sections. Climatic conditions in the central-coast region
during the growing season are characterized by mild temperatures,
usually not above 90° F, and by high fog that frequently persists until
9 or 10 a.m. during May and June and sometimes into the early part
of July. In the interior valley and in Lake and Mendocino counties,
where some of these experiments were conducted, the temperatures are
higher than in the central-coast region, and there are no morning fogs
during the growing season. In general, the amount of water required
for transpiration by the trees in the coast region is considerably less
than in the interior districts. Winter rainfall, ordinarily, is heavier in
the coast region, but in both regions there is little or no rain from
April until October or November. The heaviest rains generally occur
during November, December, January, and February.
Judging from the literature on the irrigation of pears and apples,
comparatively little work has been done with these fruits in recent years.
The irrigation of pears and apples has been studied experimentally in the
Pacific Northwest, and some reports on the irrigation of apples under
eastern conditions have been published by the United States Department
of Agriculture.
Aldrich, Lewis, and Work {1, 2, 3, 4, 5, 13, 14), l in papers dealing with
the irrigation of Anjou pears, report that the growth of fruit began to
slow down before the permanent wilting percentage5 was reached, and
while there was 60 to 80 per cent available water, or the water between
the moisture equivalent and the permanent wilting percentage, left
in the soil. Their work was carried on with a shallow, heavy, poorly
drained soil classified as Meyer clay adobe. They recommended frequent
irrigation to keep the moisture supply above 60 per cent of available
1 Received for publication April 4, 1941.
2 Pomologist in the Experiment Station.
3 Professor of Irrigation and Irrigation Engineer in the Experiment Station.
4 Italic numbers in parentheses refer to "Literature Cited" at the end of this paper.
5 In this bulletin such terms as "readily available moisture," "permanent wilting
percentages," and "moisture equivalent," are used as denned in Extension Circular
50 (18).
[3]
4 University of California — Experiment Station
water in order to maintain a normal rate of growth of fruit. Work (20)
stated that "soil moisture becomes less readily available to pear trees as
the moisture in the soil declines from the field capacity toward the wilt-
ing percentage." Work and Lewis (21) explain the necessity for main-
taining the soil moisture at a high level on the basis that (1) "the roots
do not seem to occupy the entire soil mass" and (2) "the soil moisture
content of the soil in contact with the feeding roots may be at or near
the permanent wilting percentage, while at the same time the moisture
content at some distance, perhaps only a few millimeters away, may be
much higher, thus allowing the average content for an ordinary soil
sample to be well above the wilting percentage at the time a tree shows
serious distress for water." Aldrich (1) thinks that "specific results such
as set of fruit or fruit growth in relation to a definite amount of available
moisture can be directly applied only to Anjou trees in shallow clay
adobe soil."
Hendrickson and Veihmeyer (12) reported results with Bartlett pears
in California on a Dublin clay adobe soil substantially the same as those
obtained with pears and with other fruits on medium-textured soils.
Magness, Degman, and Furr (15), working with apples in Maryland,
found no reduction in rate of growth of apples until the soil moisture in
the driest part of the root zone was near the permanent wilting percent-
age. In earlier reports, Furr and Degman (7) and Furr and Magness
(8) had concluded that there was a slight slowing down of growth of
this fruit while the moisture content was slightly above the wilting per-
centage.
Boynton (6), working with apples on a shallow soil in the Hudson
Valley, found that growth of fruit decreased when the soil moisture in
the top 2 feet was decreased to the permanent wilting percentage.
The effect of soil moisture on the quality of the fruit and upon its
keeping properties has received considerable attention during the past
few years. Haller and Harding (9), working with apples, reported that
differences in soil moisture had no effect on susceptibility to decay, but
that irrigated apples were softer and showed greater breakdown after
removal from storage. They state, however, that "the benefits from the
greater yields and higher quality of the apples grown under ample soil
moisture far outweigh the detriment of shorter storage life." Ryall and
Aldrich (16) and Ryall and Reimer (17) report higher pressure tests
and an increased proportion of total solids in pears from unirrigated
trees, but the texture and flavor of sound fruit were approximately the
same from both treatments when the fruit was in prime eating condition.
The California Agricultural Experiment Station has underway sev-
eral projects dealing with the irrigation of deciduous fruit trees. The
Bul. 667] Irrigation Experiments with Pears and Apples 5
results obtained with peaches and prunes have been published (10, 11).
This report describes similar experiments with pear trees in the central-
coast, north-coast, and Sacramento Valley regions ; and with apples in
the central-coast region. Experiments with Bartlett pears were carried
on in the E. E. Nutting orchard between Hollister and San Juan Bau-
tista in 1933 and 1934; in the H. W. Benson orchard at Kelseyville in
1935, 1936, and 1937 ; in the H. 0. Cleland orchard in Potter Valley in
1936, 1937, 1938, and 1939 ; and in the H. G. Prost orchard near Concord
in 1939; and with Bartlett and Winter Nelis pears in the University
Farm orchard at Davis in 1934, 1935, and 1936. Experiments were car-
ried on with Gravenstein apples in the William Hotle orchard at Sebasto-
pol in 1930; and with Yellow Newtown apples near Watsonville in the
Pajaro Valley, in the A. N. Jerenich orchard in 1932, and in the J. H.
Rowe orchard in 1933 and 1934.
METHODS
The various plots of trees used in these experiments were chosen for
experimental work in order to secure data from widely separated dis-
tricts having different climatic conditions and soils of different origins,
textures, and water-holding characteristics. They were laid out with the
simple objective of studying the growth, maturity, and keeping quality
of the fruit when one plot was supplied with readily available soil mois-
ture and another was left unirrigated until certain responses were ob-
tained in appearance of foliage or fruit or until the soil-moisture record
showed that water was needed. In other words, one plot was kept con-
tinuously supplied with readily available moisture, and the other was
allowed to deplete the soil moisture to the permanent wilting percentage
during certain periods. Except for a few weeds in some cases, the
orchards were essentially clean-cultivated during the summer. In a few
cases, the soil moisture in the unirrigated plots did not reach the perma-
nent wilting percentage. As the size of the fruit was expected to be re-
duced in the dry treatment, it would not have been fair to ask any
grower to sacrifice part of the returns from any considerable number of
trees. The work, therefore, was carried out in small plots without repli-
cations. All experimental trees were surrounded by guards that received
the same treatment as the experimental ones.
This type of field experiment proved satisfactory for studying the
growth of fruit, the storage qualities, and the maturity as indicated by
pressure tests ; also for determining the water-holding properties of the
various soils where the experiments were conducted. Reliable results on
yields could not be obtained, however, because of the small size of the
plots and the relative brevity of each experiment. Yield data from the
6 University op California — Experiment Station
plots at Davis were unsatisfactory some years because of the severity
of pear blight.
In every case a stock supply of soil was obtained from the plot with
a large auger, the samples being taken in 1-foot depths at from six to
eight places in the plot. From this stock supply the moisture equivalents
and permanent wilting percentages were ascertained before field work
started. The moisture equivalents were found by means of a soil centri-
fuge, and the wilting percentages by growing and wilting sunflower
plants in sealed containers. All soil-moisture percentages are based
on the oven-dry weight of the soil.
The moisture equivalent corresponds closely to the field capacity, or
the moisture percentage held in the soil after the excess gravitational
water has drained away and after the rate of downward movement of
water has materially decreased, which usually takes place within 2 or
3 days in pervious soils of uniform structure and texture. The difference
between the moisture equivalent and the permanent wilting percentage
shows what percentage of moisture is readily available to plants, and the
ratio between them is a method of indicating the proportion of moisture
that is readily available.
Soil sampling ordinarily was started in April and was continued at
one- or two-week intervals throughout the growing season. As soon as
several points were secured on a curve, showing how fast the soil mois-
ture was being used in each plot, a suitable irrigation program was
planned in cooperation with the owner. The irrigation schedule arranged
did not interfere with spraying and harvesting. The soil samples were
taken every foot to a depth of 6 feet, and for the 6-to-9-, and 9-to-12-f oot
depths. All samples were dried 4 days at 105° C.
There was close agreement between the permanent wilting percentages
and the moisture contents at which the moisture-extraction curves be-
came horizontal or nearly so. These lines should not be confused with
the relatively short, horizontal portions of the moisture curves that some-
times occurred at comparatively high moisture contents.
The soil-moisture data are graphically shown by plotting the soil-
moisture contents, expressed as percentages of the dry weight of soil,
against time. One chart for each orchard is given. In cases where the
experiments extended over more than one year, the soil-moisture data
not shown in the charts are tabulated.
The average soil-moisture contents are indicated by circles on the
charts. Straight lines were fitted, by eye, to these points, allowing a
tolerance of about 1 per cent of soil moisture. If straight lines connect-
ing several points could not be drawn within this limitation, the indi-
cated moisture contents were connected with lines from point to point.
Bul. 667] Irrigation Experiments with Pears and Apples 7
Where the positions of the points indicated wide variability in soil tex-
ture, the lines were also drawn between consecutive points. This method
of drawing the moisture curves as straight lines, within the tolerance
previously mentioned, is believed to illustrate more nearly the actual
soil-moisture conditions than where the curves are drawn to connect the
points. The tolerance of 1 per cent of soil moisture is less than the differ-
ence frequently obtained between consecutive individual samples at the
same depths.
Growth of the fruit was obtained by measuring the circumferences of
100 specimens in each plot with a steel tape each time the soil samples
were taken. The volumes, assuming the fruits to be spherical, were then
calculated. Preliminary experiments indicated that comparisons based
on volumes so calculated correspond very closely to those based on vol-
umes determined by displacement.
At harvest time, one or two boxes of fruit were obtained in each of
two pickings from each plot and taken to Davis for storage tests. On
arrival in Davis, usually the day after picking, pressure tests of each
lot were taken with a %6-inch pressure tester for pears and a %6-inch
tester for apples. These pressure tests indicate the maturity of the fruit.
Each l'ot of pears and Gravenstein apples was then divided, half being
stored at 32° F and half at 45° to 50°. At the end of 2 weeks, the fruit
was removed from storage and kept at room temperature, 70°, for
ripening. Frequent pressure tests and color comparisons were made
during the storage and ripening periods. When fully ripe the various
lots were sampled for quality. The Yellow Newtown apples were kept
in 36° storage throughout the storage period and tested at intervals for
firmness and for internal browning.
RESULTS WITH PEARS
Nutting Orchard. — The Bartlett pear trees in the Nutting orchard
were on French-pear rootstock, about forty-five years old, planted 20
feet apart, and remarkably uniform and productive. The soil was classi-
fied as a Yolo clay loam, medium to heavy in texture in the top 6 feet
and sandy in the 6-to-12-foot depth. During the experiment, layers
of different kinds of soil not apparent in the regular soil samples
were found. These layers made difficult the interpretation of soil-
moisture conditions (19). The plots consisted of five adjacent trees sur-
rounded by guard trees that received the same irrigation treatment as
the experimental ones. Soil drainage was excellent, at least to a depth
of 12 feet. Climatic conditions were mild, with morning fogs frequently
persisting until 9 or 10 o'clock. The annual rainfall was between 20 and
25 inches.
30
25
20
15
10
FIRST
FOOl
r
^IRRIGATED -*^/
UNIRRIGATED^T <? 6 .
i
RWP-10.8*
rt~- t
SECOND FOOT
20
15
10
35
30
25
20
15
THIRD
FOOT
A-
~T
Tr\
_— -o
T T
pwp.= 94*
30
25
20
15
10
30
25
20
15
10
6 TO 9 FEET
(
> — J
1 — ,
— J
\ .S
1
1
^^V-i
Pf-J
l-\
pW.p=74^
9 TO 12 FEET
T 9— -9—
RWP-6.5i;
II ""
MAY
A-y— -v — -V
I 14 29 13 27 3 10 29
JUNE JULY AUG
Fig. 1. — Soil-moisture conditions in the Nutting orchard, San Juan Bautista,
1933. The permanent wilting percentage is indicated by the letters P.W.P. The
wet plot was irrigated June 27, July 7, and July 23.
Bul. 667]
Irrigation Experiments with Pears and Apples
9
Table 1 gives the soil-moisture characteristics in the Nutting orchard.
The ratios between the moisture equivalents and the permanent wilting
percentages are fairly high, which indicates that a relatively large pro-
portion of water held by the soil at the moisture equivalent or field
capacity is readily available for use by the trees.
Figure 1 gives the soil-moisture record for the Nutting orchard in
1933. As shown by the rather flat slope of the curves, moisture was used
TABLE 1
Moisture Equivalents and Permanent Wilting Percentages
of Yolo Clay Loam Soil from the Nutting
Orchard, San Juan Bautista
Depth
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
PW.P.
feet
0 to 1
per cent
25.0
27.8
29.1
33 9
24.6
24.3
17.5
15.3
per cent
10.8
11.8
12.3
14.6
10.0
9.4
7.4
6.5
ratio
2.31
1 to 2
2.36
2 to 3
2.37
3 to 4
2.32
4 to 5
2.46
5 to 6
2.59
6 to 9
2.36
9 to 12
2.35
slowly ; at no time during the season between May 11 and August 24,
when the second picking was made, did the soil moisture in either plot
reach the permanent wilting percentage. Furthermore, the covercrop
of native weeds, present on both plots and used as indicator plants,
showed no wilting even on warm afternoons. The wet plot was given
three irrigations. The first was applied on June 27, but only penetrated
to a depth of about 3 feet, the second was applied on July 7, and the
third on July 23. After each irrigation there was more moisture in the
top 3 feet in the wet plot than in the dry. Since several days, sometimes
a week or more, elapsed between the times samples were taken before and
after irrigation, the curves for the Nutting, as well as the other orchards,
do not indicate the moisture conditions during these periods. They do,
however, give a general picture of the moisture conditions and show
that the irrigated plots were at a much higher moisture level than the
unirrigated ones. From early in May until the middle of June, the soil-
moisture conditions in the dry plot were higher than in the wet, but
thereafter they were higher in the top 3 feet of the wet plot than in the
dry, and approximately equal in the second 3 feet. The soil-moisture
record for the 6-to-9-foot depth is so erratic, probably owing to the vari-
10
University of California — Experiment Station
ation in soil texture, that no attempt is made to interpret it. Little or
no moisture was extracted from the soil in the 9-to-12-foot depth.
In 1934 the soil-moisture results resembled those of 1933. The trees
in both plots used water slowly and did not exhaust the readily avail-
able supply during the growing season. The wet plot was irrigated on
May 9 and on June 15. Table 2 shows the moisture percentages for 1934.
TABLE 2
Average Soil-Moisture Percentages, Nutting Orchard, San Juan Bautista, 1934
Date
0-1 ft.
-2 ft.
2-3 ft. 3-4 ft. 4-5 ft. 5-6 ft. 6-9 ft. 9-12 ft.
Wet plot'
April 18
May 3
May 16
May 31
June 12
June 28
July 12
July 25
August 24
September 21
October 25 . . .
April 18
May 3
May 16
May 31
June 12
June 28
July 12
July 25
August 24
September 21
October 25 . . .
per cent
per cent
per cent
per cent
per cent
per cent
per cent
21.1
16.8
18.2
21.8
15.8
13.3
11.9
12.4
16.0
17.2
19.3
15.6
12.8
21.0
26.1
26.8
25.2
21.3
18.5
20.0
25.4
28.6
24.5
18.7
15.0
19.1
24.8
27.2
23.3
15.3
15.3
21.1
29.2
21.0
34.0
31.1
29.5
16.9
25.4
29.3
31.7
28.8
27.9
17.2
16.2
24.1
28.7
29.9
27.7
28.1
14.8
21.5
22.9
25.2
19.0
24.3
15.2
19.8
21.2
24.1
21.5
20.8
13.9
17.8
20.0
21.8
18.9
16.0
16.0
per cent
13.8
13.2
14.8
Dry plot
per cent
per cent
per cent
per cent
per cent
per cent
per cent
12.4
16.1
25.7
22.7
13.4
12.3
12 5
12.3
15.0
16.4
20.2
16.4
13.4
11.6
15.9
16.9
19.4
14.1
13.3
14.4
17.6
18.3
21.5
16.4
15.6
13.8
16.9
18.7
21.5
16.9
15.8
12.6
16.1
17.8
19.3
15.7
11.4
11.7
16.0
16.5
19.5
12.0
12.3
10.8
11.4
16.5
17.6
19.9
12.1
12.6
11.7
16.4
16.9
19.1
13.2
12.3
10.2
16.0
17.1
19.8
14.1
12.8
11.1
15.5
16.8
18.2
13.8
10.9
8.9
per cent
18.4
16.4
* Wet plot irrigated May 9 and June 15.
The volumes of the measured fruits in 1933 are given in figure 2. Since
there were no consistent differences in the growth of the fruits, evi-
dently the trees in both plots secured a supply of water at all times.
Results in 1934 were similar. Evidently, therefore, the growth of the
fruit was not affected by the differences in soil moisture found under
the conditions existing in the Nutting orchard.
At harvest time in 1933, two pickings were made, one on August 10
and the other on August 24. Two boxes of fruit were secured from each
plot at each picking and brought to Davis for storage tests. In both pick-
Bul. 667]
Irrigation Experiments with Pears and Apples
11
ings the pressure tests of the fruit from the irrigated plot were slightly
but not significantly higher than those from the dry, but these differ-
ences disappeared during storage. The fruit stored at 50° F and that
stored at 32° kept equally well in storage, and no significant differences
were found in subsequent pressure-tests, the color, or the eating quali-
100
JUNE
Fig. 2. — Sizes of pears in the Nutting orchard, 1933.
ties. In 1934, when fruit samples were picked on July 20 and 25, similar
results were obtained. The initial pressure tests for both years were as
follows :
Pressure test, wet Pressure test, dry-
Date picked plot, in pounds plot, in pounds
August 10, 1933 16.7 16.3
August 24, 1933 15.4 14.9
July 20, 1934 18.1 17.1
July 25, 1934 17.3 16.5
Benson Orchard. — The Bartlett pear trees in the Benson orchard were
on French-pear rootstock, about thirty years old, planted 24 feet apart,
and fairly uniform in size. Except for the irrigation treatment, the cul-
tural operations followed the general practice of the district. The soil,
classified as a Dublin clay adobe, contains about 50 per cent clay and
exhibits typical adobe structure. This orchard is situated some 2 miles
from Clear Lake near Kelseyville at an elevation of perhaps 1,300 feet
above sea level. Each spring, during 1935, 1936, and 1937, the water
table rose to approximately 4 feet below the surface, but dropped below
6 feet as the season progressed. As far as could be observed, no deleteri-
ous results followed. Furthermore, the fruit behaved the same in the dry
plot as in the dry plots of other pear orchards where no water table was
12
University of California — Experiment Station
encountered. The plots consisted of four trees each, surrounded on all
sides by guards that received the same irrigation treatment as the experi-
mental trees. During the growing season, the climatic conditions were
characterized by clear, warm days with maximum temperatures occa-
sionally as high as 100° F. The average annual rainfall was slightly more
than 20 inches.
Table 3 gives the water-holding characteristics of this soil. The soil-
moisture record for the 1935 season is shown graphically in figure 3, as
TABLE 3
Moisture Equivalents and Permanent Wilting Percentages
of Dublin Clay Adobe Soil from the Benson
Orchard, Kelseyville
Depth
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
P.W.P.
feet
0 to 1
per cent
26.0
28.1
27.0
24.4
22.6
25.6
per cent
13.6
17.3
16.5
15.0
15.8
18.0
ratio
1.91
1 to 2
1.62
2 to 3
1.64
3 to 4
1.63
4 to 5
1.43
5 to 6
1.42
a typical example of the results during the three years of experimental
work in this orchard. According to the data for 1935, the soil-moisture
conditions were about the same in both plots from the last of April until
the last week of June. The wet plot was irrigated on June 27 and on
July 24, or shortly after the soil samples were taken in the latter parts
of June and July. These irrigations penetrated to a depth of about 4
feet. The dry plot received no irrigation water while the fruit was on
the trees. The permanent wilting percentage was reached in the top 4
feet of the dry plot about July 9, as indicated by the flattening of the
curves ; but there was readily available moisture in the fifth and sixth
foot throughout the season. Some water, however, was used from these
depths during the season, as shown by the downward slope of the curves.
Because of the topography of the region, standing water was encountered
at a depth of about 4 feet during the winter and early spring. As the
water table gradually receded, the trees were able to reduce the moisture
content of the soil in the fifth and sixth foot to a certain extent ; but in
the dry plot the amount that they thus obtained was apparently not
sufficient to keep the fruit growing at a normal rate when the top 4
feet of soil was dry. In this particular orchard the growth of the fruit
depended largely upon the moisture in the top 4 feet.
Bul. 667"
Irrigation Experiments with Pears and Apples
13
Figure 4 shows the growth of the fruit in the Benson orchard in 1935.
The average sizes between May 14 and June 25 were about the same in
both plots ; but after the latter date, the fruit grew more slowly in the
first foot
Fig. 3. — Soil-moisture conditions in the Benson orchard, Kelseyville, 1935. The
permanent wilting percentage is indicated by the letters P.W.P. The wet plot was
irrigated June 27 and July 24.
dry plot than in the wet. The decreased growth in the dry plot occurred
at about the time the moisture content in the top 4 feet reached the
permanent wilting percentage.
Table 4 gives the soil-moisture data for the Benson orchard for 1936
and 1937. The wet plot was irrigated on July 3 and July 27 in 1936 ; on
14
University of California — Experiment Station
TABLE 4
Average Soil-Moisture Percentages, Benson Orchard, Kelseyville,
1936 and 1937
Wet plot*
Dry plot
Date
0-1
ft.
1-2
ft.
2-3
ft.
3-4
ft.
4-5
ft.
5-6
ft.
0-1
ft.
1-2
ft.
2-3
ft.
3-4
ft.
4-5
ft.
5-6
ft.
1936:
April 28
per
cent
19.3
26.3
22.1
26.7
18.7
26.8
21.5
22.8
17.7
15.1
14.1
14.4
24.7
22.7
16.8
20.9
20 9
17.7
per
cent
19.8
20.4
18.3
21.5
18.0
22.5
20.5
22 0
20 0
15.2
18.1
16.9
21.6
21.7
19.0
21.6
22.1
19.6
per
cent
19.8
18.5
17.0
18.2
15.9
19.8
18.2
19.7
16.9
14.6
19.1
17 1
18.6
20.1
17.6
19.2
19.9
16.7
per
cent
20.8
19.9
19.3
18.2
17.7
19.7
17.5
18.4
15 0
14.3
20 4
18.7
17.6
19.3
15.4
17.0
19.3
15.7
per
cent
21.9
22.7
21.8
21.9
22.1
19.9
19.4
20 2
17.4
16.8
21.6
20 9
20 0
20 2
17.7
18.5
17.8
16.3
per
cent
23.0
22.3
23.5
23 0
23.3
22 0
21.8
22 0
21.6
19.7
22.9
22.5
22.9
24 5
22.8
21.1
22.6
19 1
per
cent
19.1
25.2
22.5
27.8
20.2
16.6
15.1
13.9
12 5
16.2
15.5
13.8
17.8
15 5
12.9
13.2
12.8
per
cent
23.9
24 3
22.7
23.5
21.3
18.9
18.3
17.3
16.7
16.0
22.4
20 8
19 5
20 0
18.9
18.4
17.1
16.2
per
cent
22.8
22 4
21.6
20.7
19.6
17.7
16.7
16.3
16 0
17 0
21.1
18.4
19.4
19.2
17.9
17.8
16.8
15.7
per
cent
20.5
21.5
20.9
21.6
20.2
18.4
15.2
14.8
15.0
13.8
20 5
20.6
19.2
17.2
16.1
15.2
15.3
14.1
per
cent
21.4
21.8
21.9
23.4
23 0
20.1
17.1
15.8
15 9
15.8
20.9
20.5
20.6
19.5
18.5
16.9
16 7
15.0
per
cent
23.0
May 12
21 8
May 26
24.5
25.8
June 23
25 4
July 7
22 1
July 21
20.4
August 4
August 18
September 22
19.2
18.0
17.6
1937:
May 18
25.6
23.3
22 1
June 29t
22.7
July 13
21.7
July 27
19.7
18.2
17.7
* Irrigated July 3 and July 27, 1936; and June 8 and July 14, 1937.
f Rain before sampling.
!0
IOO
90
80
70
60
50
40
30
20
10
/
/*
4&
^
o
14 28
MAY
II 25
JUNE
9 23
JULY
6 13
AUG
Fig. 4. — Sizes of pears in the Benson orchard, 1935.
Bul. 667] Irrigation Experiments with Pears and Apples 15
June 8 and July 14 in 1937. In addition, both plots were irrigated May
9, 1936, in order to disk the covercrop. The irrigation dates varied from
year to year because of the necessity of avoiding conflicts with the spray
program. The depths of penetration for the different irrigations are
indicated by the increased moisture contents of the top 3 or 4 feet of soil
on the sampling dates immediately after the irrigations. A rain of 2.2
inches occurred between June 1 and 8 in 1936, and one of 1.2 inches
occurred on June 15, 1937. According to the data, the rate of extraction
of moisture by the trees in the dry plot began to decrease materially
between July 21 and August 4 in 1936 and between July 27 and August
10 in 1937. In other words, the permanent wilting percentage was reached
later in the season, both in 1936 and 1937, than in 1935.
The sizes of pears from both plots for 1936 and 1937 are given in table
5. These data when plotted give curves substantially similar to the one
for 1935, which is given in figure 4. In all cases in the Benson orchard,
the fruit from both plots increased at about the same rate so long as
readily available moisture was present, but the fruit in the dry plot
grew more slowly than that in the wet plot when the readily available
moisture in the top 4 feet of soil in the former was about exhausted.
At harvest two pickings of two boxes from each plot were taken to
Davis for storage tests. Each picking was divided into two lots, one being
stored at 32° F, the other at 50°. The fruit was tested with the standard
pressure tester on arrival and at frequent intervals thereafter. The dates
of picking and the pressure tests of the pears upon reaching Davis were
as follows :
Pressure test, wet Pressure test, dry
Date picked plot, in pounds plot, in pounds
August 6, 1935 21.4 24.2
August 13, 1935 22.5 25.3
August 4, 1936 18.6 18.6
August 18, 1936 17.2 17.5
August 10, 1937 26.0 25.4
August 24, 1937 23.6 25.5
The pressure tests showed different results in 1935 than in 1936 and
1937. In 1935, when the fruit was picked 3 or 5 weeks after the soil in
the dry plot was reduced to the permanent wilting percentage, the fruit
from the dry plot was harder than that from the wet one. No significant
differences were found in 1936 in either picking in the pressure tests,
although the fruit started to slow down in growth July 21. In 1937 the
fruit from the wet plot was slightly but not significantly harder than
that from the dry at the first picking, but 2 weeks later the fruit from
the dry plot was significantly harder than that from the wet. Apparently,
if the soil moisture is reduced to the permanent wilting percentage long
16
University of California — Experiment Station
enough before picking, the unfavorable soil-moisture conditions are
reflected in an increase in the pressure tests.
In the storage tests, if the fruit from the dry plot was harder than that
from the wet, it tended to remain so while kept at 36° F. After removal
to room temperature, however, the pressure differences disappeared,
and fruit from both plots reached prime eating condition at about the
same time. The stored fruit ripened more rapidly at 50° than at 36°,
and the differences in pressure tests diminished. Color changes pro-
TABLE 5
Average Volumes of Pears, Benson Orchard, Kelseyville, 1936 and 1937
1936
193;
Date
Wet
Dry
Date
Wet
Dry
May 12...
May 26...
June 9. . . .
June 23...
July 7...
July 21...
July 29...
August 4 .
August 18
3.6
9.9
17.8
32.0
52.3
78.0
94 0
106.5
136 0
cc
4.4
11.0
19.5
34 5
53 0
71.9
81.6
88.1
102 2
May 17. .
June 1
June 14. . .
June 29 . .
July 13 . . .
July 27...
August 10
August 24
2.9
9.0
17.8
20.6
42 2
61.6
80.7
92.0
cc
3.3
9.6
19.9
33.5
47.1
61.6
70.3
73.7
gressed at about the same rate in the fruit from both treatments. At
maturity the only distinguishing characteristic was the somewhat smaller
size of the fruit from the dry plot.
Cleland Orchard. — The Bartlett pear trees in the Cleland orchard
were on Japanese-pear rootstock, about twenty years old, and planted
20 feet apart. The orchard was situated in Potter Valley, Mendocino
County, on the bank of the east fork of the Russian River. Except during
flood periods, the level of this stream was about 20 feet below the ground
level of the experimental plots. The soil was classified as a Yolo fine
sandy loam, stream-bottom phase. About 2 feet beneath the surface, a
layer of fine-textured soil made sampling difficult at times, but did not
seem to interfere with drainage. The climatic conditions resembled those
in the Benson orchard at Kelseyville ; but the annual rainfall was con-
siderably more, averaging 30-35 inches.
Two plots of six trees each were used, and the same general procedure
was followed as with the other plots under experimentation. Table 6
gives the water-holding properties of the soil, figure 5 the soil-moisture
record for 1938. After the heavy rains during the winter of 1937-38,
the soil in both plots was filled to field capacity to a depth of at least 12
Bul. 667]
Irrigation Experiments with Pears and Apples
17
feet. Beginning with the initial sampling on April 23, the soil moisture
was approximately equal in both plots until shortly after June 22, when
the wet plot was irrigated. A second application of water was given the
wet plot on July 20. Both irrigations penetrated to a depth of 6 feet ;
and, apparently, some water was added to the supply in the 6-to-9-foot
depth. The moisture contents of the soil in both plots were reduced to
about the permanent wilting percentage on June 22 in the top 4 feet.
TABLE 6
Moisture Equivalents and Permanent Wilting Percentages
of Yolo Fine Sandy Loam Soil from the Cleland
Orchard, Potter Valley
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
Depth
P.W.P.
feet
0 to 1
1 to 2
2 to 3
3 to 4
4 to 5
5 to 6
6 to 9
9 to 12
per cent
16.0
15.8
19.8
22.9
21.5
20.4
17.7
16.9
per cent
6.7
8.5
10.9
12.0
11.5
11.1
7.6
7.7
ratio
2.39
1.86
1.82
1.91
1.87
1.84
2.33
2.19
In the wet plot this condition prevailed only until June 27, when water
was applied ; but in the dry it lasted through the remainder of the grow-
ing season. Furthermore, the available moisture in the dry plot was ex-
hausted in the fifth foot on July 6, in the sixth on August 3. The trees
in the dry plot could not secure enough water from the available supply
below 6 feet to maintain a normal growth of fruit, as is shown later
(fig. 6). Apparently more water was extracted from the 6-to-9-foot
depth in the dry plot than in the wet, whereas in the 9-to-12-foot depth
the extraction was about equal in both plots.
According to the soil-moisture curves, under conditions prevailing in
the Cleland orchard, with the soil at field capacity at the beginning of the
season, the trees use the water fast enough to necessitate a first irriga-
tion between the middle and the last of June. At that time the soil mois-
ture resulting from the winter rains is exhausted to a depth of 4 or 5 feet,
and the supply should be replenished in order for the tree to function
normally.
Figure 6 gives the average volumes of 100 pears in each plot in 1938.
The fruit in both plots grew at about the same rate from May 10 to June
22, when the pears in the dry plot began to grow more slowly than those in
FIRST FOOT
•**-
SECOND
FOOT
fot'^
9-^
RW.P= 8.5-*
_ ....
11
Lt— {
r~r-H
THIRD FOOT
FOURTH
FOOT
>
"^*^
^7^*7
/
PWP=J202
s
r v 4r — 4-4
FIFTH FOOT
10
5
20
15
10
5
15
10
5
25
20
.5
O 10
25
° 20
2 15
j< 10
u 25
U
°- 15
10
25
20
15
10
25
20
15
PWP=_77%
23" "T6" 25 8 22 6 20 3 10
APRIL MAY JUNE JULY AUG
Fig. 5. — Soil-moisture conditions in the Cleland orchard, Potter Valley, 1938.
The permanent wilting percentage is indicated by the letters P.W.P. The wet plot
was irrigated June 22 and July 20.
••^
***4
^v
PWP=li5_2
rH
r (
r 1
?"9
SIXTH FOOT
c
r* ■|
^.^
PWP=
RU
**•<
>—. <
J—
W
5 TO
9 FEET
. — <
~-
r
PWP=762
9 TO
12
FEET
1
>
Bul. 667] Irrigation Experiments with Pears and Apples
19
the wet. The differences in size increased until August 9, when the crop
was harvested. Just as in the Benson orchard, the slow growth of fruit
in the dry plot started about the time when the readily available moisture
was exhausted in that plot.
The soil-moisture results in 1936, 1937, and 1939 (table 7) resembled
those in 1938. In each of these years the soil moisture was approximately
at field capacity when the first samples were taken, about the middle of
May, except in the top foot, where some moisture had been lost by evapo-
120
110
100
90
80
U 70
^ 60
CD
3 50
? 40
30
20
10
/
/ /
/
/ /
/ s
/ *
i
10 24
MAY
8 22
JUNE
6 19 2 9
JULY AUG
Fig. 6. — Sizes of pears in the Cleland orchard, 1938.
ration from the surface and by transpiration from the cover crop. The
moisture supply in both plots was increased by a rain of 2.7 inches during
the first week in June in 1936. The wet plot was irrigated July 8 in 1936
and again 10 days later, the soil being wet to a depth of more than 6 feet.
In the wet plot the soil moisture was above the wilting percentage while
the fruit was on the trees, but the wilting percentage was reached in the
dry plot in the top 4 feet about July 22 in 1936.
In 1937 the dry plot was accidentally irrigated on June 8, while the
wet plot was being watered, and, with a rain of slightly more than an inch
on June 15, was wetted to a depth of about 6 feet. The wet plot was
irrigated a second time on July 23. The readily available moisture in
the dry plot was almost exhausted in the top 4 feet on July 28, or about
a week before the first picking of 1937. The accidental irrigation on June
8 supplied this plot with sufficient moisture to maintain the available
supply about a week longer than in 1936. As wTas the case in 1938, some
TABLE 7
Average Soil-Moisture Percentages, Cleland Orchard, Potter Valley,
1936, 1937, and 1939
Date
0-1 ft.
1-2 ft.
2-3 ft. 3-4 ft.
4-5 ft.
5-6 ft.
6-9 ft.
9-12 ft.
Wet plot*
1936:
May 13
May 27
June 10f. •
June 24 . .
July St....
July 15
July 22
July 29
August 19. .
October 13.
1937:
May 19
June 2
June 151 •
June 30 ... .
July 14
July 28
August 4. . .
August 11. .
October 21 1
1939:
April 26..
May 11....
May 25t
June 8
June 22. . .
July 6
July 20
August 3. . .
August 17. .
October 13.
14.7
14.4
19.7
13.9
18.1
19.2
12.4
10.0
7.6
11.7
8.6
21.5
15.7
8.4
19.7
17.8
12.9
18.4
9.6
10.0
15.5
12.7
19.1
12.1
17.9
11 3
9 2
9.1
19.1
19.7
17.6
12 7
16.3
18.8
15.8
12.6
17.3
12.2
21.7
19.0
12.2
22.7
19.7
18.2
11.3
20 6
15.8
12.1
10.3
18.3
15.8
16.1
14 1
15.8
17.5
16.7
13.7
10.0
17.8
14.8
21.7
18.4
15.9
20.1
18.8
19.3
13.9
18.6
14.6
11.2
13.3
19.6
17 1
20 2
15.8
12.8
11.2
19.3
16.7
16.3
16.8
18.3
19.0
17.7
17 5
10.6
18.9
17.8
20.3
17.9
16.5
20 0
19.0
19.6
13.4
19.8
18.5
13.8
14.3
20 3
18.9
21 2
16.4
13.1
12.8
20.5
18.6
17.4
16 3
18.7
17.6
18 2
14 9
11.0
19.6
17.6
20.7
18.8
16.6
20 0
18.6
18.2
13.7
21.6
18.4
15.8
15.3
19.3
17.2
20.6
16 5
14 4
10 9
20 0
19.6
18.9
16.8
16 8
17.4
17 1
17.7
18.3
18.0
— —
14.7
— — -
10.8
13.0
19.2
16.8
18.3
20 .1
20 5
19.3
17.6
21.0
17.8
18.2
14 2
15 5
20.9
19.6
17 6
16.5
17.3
16.0
17.6
20.9
18.1
17.4
20 1
19.2
18.4
— —
15.3
16.7
10 5
11.9
20.4
18.3
19.2
15.2
18.2
19.4
22 0
*17.7
17.1
15.7
17 5
14.5
13.3
Dry plot
1936:
May 13
May 27
June 10t.
June 24 ... .
JulyS
July 15
July 22
July 29
August 19. .
October 13 .
1937:
May 19
June 2
June 15t§. •
June 30
July 14
July 28
August 4 . . .
August 11 .
October 21 f
1939:
April 26... .
May 11
May 25t
June 8
June 22. .
July 6
July 20
August 3. .
August 17. .
October 13 .
12.4
13.4
20.2
12.1
9.1
8.2
7.7
7.4
6.0
6.1
9.8
18.
13.
10
8
7
7.
14.1
7.2
14.7
11.2
8.7
7.8
7.1
5.9
5.8
6.6
16.7
16.7
20.3
19.5
20.1
19.4
15.2
16.5
16.7
18.2
19.3
17.2
15.1
16.2
17.2
18.1
14.8
14.7
14 9
15 4
16.1
17.9
10.5
12.3
13.8
12.7
14.9
9.4
12.0
13.3
13.8
15 3
.
9.5
11.0
12.5
13.0
13.2
13 4
8.5
10.9
12.2
11.7
12 5
. — ■ — -
9.0
11.8
12 4
12.4
11.8
8.5
10 4
11.0
10.7
11.1
11.5
15.1
19.7
20 2
20.1
20.1
11.0
16.2
17.9
18.5
18.6
19.3
22.6
21.3
20.3
19.3
19 7
17.8
20.9
18.9
18.9
19.2
19.3
14.2
15.2
17.0
16.3
17.7
— . — ■
9.8
12.2
13.8
14.7
17.3
9.0
11.7
13.2
13.4
15.8
9.4
12.1
13.2
12.9
14.2
— —
9.2
10 7
12.7
12.2
12.1
14.8
13.8
19.1
20.5
19.3
19.1
18.9
10.4
14.4
17.3
18.2
17.3
7.8
11.3
14.2
15.6
17.3
18.3
10.4
13.5
15.7
16.2
17.1
17.6
9.7
11.7
13 0
15.0
15.9
8.9
11.2
12.9
12.6
14.0
15.7
9.1
12.7
12.7
13.0
14.1
15.1
8.2
10.1
11.7
12.3
12.8
7.8
10 5
11.1
10.1
11.0
12.8
7.9
9.7
10 6
10.1
9.9
10.3
21.7
19.3
17.3
15.5
19.6
19.9
18.8
17.2
18.7
14.3
11.9
* Irrigated July 8 and July 18 in 1936; June 8 and July 23 in 1937; and June 17 and July 18 in 1939.
t Rain before sampling.
t Wet plot being irrigated, could not be sampled.
§ Accidental irrigation before sampling.
Bul. 667]
Irrigation Experiments with Pears and Apples
21
water was removed from the 6-to-9-foot depth, but very little from the
9-to-12-f oot depth ; but the amount thus secured did not keep the fruit
in the dry plot growing so fast as that in the wet one. In 1939 both plots
had about the same moisture conditions from late in April until June 17
when the wet plot was irrigated. A second irrigation applied to the wet
plot on July 18 kept the soil moisture in the top 6 feet above the perma-
nent wilting percentage until after the crop was picked. The soil mois-
TABLE 8
Average Volumes of Pears, Cleland Orchard, Potter Valley,
1936, 1937, and 1939
1936
1937
1939
Date
Wet
Dry
Date
Wet
Dry
Date
Wet
Dry
May 13
cc
5.0
11.3
20.2
36.1
73.6
87 2
99.1
cc
5.4
12.0
21.3
37.7
56.5
71 .9
82.5
91.0
May 19
June 1
June 14
June 29
July 13
July 27
cc
3.5
10 3
21.7
40.5
63.1
9G 0
115.4
132.2
cc
3 5
10.1
22.1
39.9
62.4
87.2
100.0
109.7
May 11
cc
5.6
12.5
24.4
42.2
67.8
99.1
129.7
cc
5.9
May 27
May 25
12.8
June 10
24 8
June 24
July 8
Julv 15
June 22
July 6
July 20
41.1
63.9
85.2
July 22
July 29
August 4
August 10
August 3
101.1
ture in the dry plot was reduced to about the permanent wilting
percentage in the top 4 feet on July 6.
Table 8 gives the fruit sizes for 1936, 1937, and 1939. When plotted,
these data show that growth of the fruit in the dry plot slackened each
year near the time when the readily available moisture was exhausted
in that plot. The dates of picking and the pressure tests of the pears from
the Cleland orchard were as follows :
Pressure test, wet Pressure test, dry
Date picked plot, in pounds plot, in pounds
July 29, 1936 17.2 17.2
August 19, 1936 16.0 16.9
August 4, 1937 23.7 26.1
August 11, 1937 20.8 24.6
August 3, 1938 20.6 24.8
August 10, 1938 20.3 21.9
August 3, 1939 18.5 21.5
August 17, 1939 20.3 19.5
The fruit from the dry plot was harder than that from the wet in 1937,
in 1938, and in the first picking of 1939, but in 1936 only a slight differ-
ence in the second picking was obtained. In 1936 the available soil mois-
ture in the dry plot was not exhausted until July 22, and picking started
22
University of California — Experiment Station
Ld
D
h-
00
o
2
00
LJ
O
z
u
u
q:
u
CL
30
25
20
15
10
30
25
FIRST
FOOT
!v
1
/
N^RRIGATED
W-
/
X^
s,
I O —"T---.A
^ — A— 4
PWP=I2.5%
T ? S
'— - i — ^
SECOND FOOT
10
30
25
THIRD FOOT
206
O '5
" 10
y
X,
t^l
)-.
' 7
b ,
--J
Z
RW:P=H.8_%_
1 l, ...J
•H
^--i— A— o
30
25
20
15
I 0' RW.P=I05 %
30
FOURTH FOOT
-i-h
^
ysCT<
lTTj
r--<
prtrt
>- — —(•
li «!
l) o
FIFTH FOOT
6 19
JUNE
17 31
JULY
AUGUST
Fig. 7. — Soil-moisture conditions in the University Farm orchard, Davis, 1934.
The permanent wilting percentage is indicated by the letters P.W.P. The wet plots
were irrigated May 18 and August 10.
Bul. 667]
Irrigation Experiments with Pears and Apples
23
a week later. In 1937, on the other hand, although because of an accidental
irrigation in June the available soil moisture in the dry plot was not ex-
hausted until picking time or shortly before, markedly higher pressure
tests were secured than in the wet plot. In 1938 the trees in the dry plot
were without readily available soil moisture after June 22, and the pears
were considerably harder in this plot than in the irrigated one. In 1939
the available soil moisture was exhausted shortly after June 22 in the dry
TABLE 9
Moisture Equivalents and Permanent Wilting Percentages
of Yolo Loam Soil from the University
Farm Orchard, Davis
Depth
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
P.W.P.
feet
0 to 1
per cent
24.2
24.3
22.7
19.6
21.6
21.6
per cent
12.5
12.6
11.8
10.5
10.3
10.7
ratio
1.94
1 to 2
1 93
2 to 3
1.92
3 to 4
1.87
4 to 5
2.10
5 to 6
2.02
plot, and the pears from the first picking in this plot on August 3 were
harder than those from the wet plot.
The differences in firmness tended to disappear in storage. The pres-
sure tests of the fruit from the dry plot in the 50° F storage tended to ap-
proach those of the fruit from the wet plot more rapidly than in the 36°
storage. After removal from storage, fruit from both plots ripened
rather quickly, and when fully ripe had apparently the same quality.
University Farm Orchard. — Two plots each of the Bartlett and Winter
Nelis varieties on Japanese-pear rootstock, planted 27 feet apart, were
used in 1934, 1935, and 1936. The plots contained 10 trees each. The trees,
though planted in 1923, were small for their age because of heavy prun-
ing during several epidemics of pear blight. The soil was classified as a
Yolo loam with excellent drainage. Climatic conditions were typical of
the interior valleys. The plots were cultivated several times during the
season, and no covercrop was planted. Table 9 gives the water-holding
properties.
The curves in figure 7 indicate the average soil-moisture conditions in
the Bartlett and Winter Nelis plots in 1934. The wet plots were irrigated
on May 18 and on August 10. Rain, early in June, added some moisture
to the top foot in both plots. Since the samples were taken as soon as
24
University of California — Experiment Station
possible after irrigation, and before the water had reached the greatest
limit of penetration, the increase in moisture content in the fifth foot
occurred about a week later than it did in the upper 4 feet, and in the
sixth foot, still later. As usual, the dry plots were not irrigated during
the growing season. The permanent wilting percentage was reached
about July 3 in the top foot, about August 1 in the second, August 15 in
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
I 0
0
/ /
e
ARTLETT
■ /
/
'
y""
S
/'
WINTER
NELIS
^^~~~— ^ *''
/'/
V ■''
y
tfs
zzzS^
'^> —
30 I
Fig. 8. — Sizes of pears in the University Farm orchard, 1934.
14 28
MAY
II 25
JUNE
9 23
JULY
6 21
AUG
the third, July 17 in the fourth, and August 15 in the fifth foot in both the
Bartlett and Winter Nelis plots ; pockets of fine gravel in the sixth foot
made the results erratic at this depth. Thus the permanent wilting per-
centage was not reached in the top 5 feet until comparatively late in the
season in 1934, possibly because of cool weather and some showers during
June.
Figure 8 shows the growth of the Bartlett and Winter Nelis pears in
1934. The growth of the Bartlett pears in both plots was substantially
the same until after August 6, after which the fruit grew more slowly in
the dry plot than in the wet. The fruits of the Winter Nelis pears in the
dry plot increased more rapidly in size than those in the wet during the
early part of the season, but slowed down appreciably after August 6.
Bul. 667]
Irrigation Experiments with Pears and Apples
25
As the soil-moisture record shows, the permanent wilting percentage was
reached in the top 5 feet about the time the fruit in the dry plot began
to slow down in growth.
TABLE 10
Average Soil-Moisture Percentages, University Farm Orchard, Davis,
1935 and 1936
Date
Wet plot*
0-1
ft.
1-2
ft.
2-3
ft.
3-4
ft.
4-5
ft.
Dry plot
0-1
ft.
1-2
ft.
2-3
ft.
3-4
ft.
4-5
ft.
5-6
ft.
1935:
May 25
June 8
June 22
June 29
July 5
July 13
July 20
July 27
August 2
August 9
August 17
August 31 —
September 28
1936:
May 25
June 1
June 8
June 14
June 22
June 29
July 6
July 13
July 20
July 27
August 3
August 10 ... .
August 24 . . .
September 7 .
per
cent
16.1
23.3
17.5
15.6
14.0
13.4
12.0
22.8
21.3
19.1
13.1
11.2
18.2
16.1
16.3
15.7
13.9
13.6
27.3
21.7
16.7
17.0
14.6
13.4
25.4
19.5
per
cent
24.2
22.3
20.5
17.7
15.6
15.1
22.2
23.4
22.0
15.6
13.4
19.1
19.1
18.3
18.7
16.3
15.4
25.7
23.6
21.3
22.6
18.1
16.0
26.3
23.7
per
cent
16.4
21.5
19.5
16.8
15.5
13.4
14.6
18.6
19.0
18.2
13.0
10.5
16.5
15.3
16.9
14.4
13.6
13.1
22.3
20.4
17.4
20.1
12.5
13.3
23.1
20.3
per
cent
15.9
21.9
16.9
15.5
13.6
13.5
12.5
14.0
16.7
15.7
13.9
14.6
14.7
14.7
13.4
12.0
10.9
15.0
15.6
15.3
16.2
11.9
9.5
18.5
14.1
per
cent
17.8
25.0
20.0
18.7
17.2
16.8
14.2
14.8
18.3
16.8
15,9
11.7
16.7
15.9
16.6
15.0
13.1
11.7
15.4
14.5
16.1
16.2
12.3
10.1
17.2
15.6
per
cent
15.3
25.7
20.2
19.8
19.3
18.3
15.2
15.9
18.6
15.6
16.5
11.2
18.6
17.1
17.1
16.4
15.2
13.5
18.1
17.3
18.0
17.7
14.0
11.3
17.0
16.2
per
cent
16.8
15.1
13.7
13.0
12.4
11.9
11.6
10.8
10.2
10.3
11.0
9.8
10.1
16.7
16.0
17.5
15.4
15.3
11.3
12.4
12.4
11.8
11.5
10.2
9.4
11.6
10.4
per
cent
21.9
20.3
16.8
15.7
13.9
14.6
14.4
13.6
12.6
13.5
13.1
12.8
13.4
19.8
18.9
19.1
17.2
18.8
15.5
14.4
14.9
15.3
14.2
13.1
13.0
15.3
13.4
per
cent
19.1
19.0
14.2
13.3
12.8
13.0
12.7
11.5
11.8
12.0
11.6
10.3
12.1
19.0
20.2
17.1
17.9
16.5
14.8
13.0
13.6
13.7
14.0
13.7
12.1
12.4
9.6
per
cent
18.2
17.3
14.2
12.8
14.4
12.9
9.9
11.3
10.6
10.5
10.2
10.1
10.8
15 6
17.7
16.2
15.6
14.5
12.9
11.0
12.4
13.4
11.3
12.5
10.6
10.8
11.0
per
cent
18.0
17.9
13.5
14.3
12.7
12.5
12.2
11.3
11.6
10.8
10.5
10.1
11.4
18.6
18.0
17.9
17.1
15.4
14.1
12.9
13.2
13.4
11.6
11.9
10.7
11.7
11.0
per
cent
17.8
20.4
16.8
16.1
16.9
14.7
14.0
12.3
11.6
12.8
11.5
11.4
12.4
20.7
19.0
18.8
17.7
16.1
13.3
15.7
15.0
14.9
12.3
15.0
11.6
13.4
11.5
* Irrigated June 5 and July 28, 1935; and July 1 and August 14, 1936.
Table 10 gives the soil-moisture percentages for the plots at Davis for
1935 and 1936. The wet plots were irrigated on June 5 and July 28 in
1935; on July 1 and August 14 in 1936. In the dry plots in 1935 the data
plotted show a marked change in the slope of the extraction curves
about June 29 in the first foot ; July 5 in the second and third ; July 13
in the fourth foot ; August 9 in the fifth ; and August 17 in the sixth.
The moisture content at which the change in slope of the extraction
curves occurred coincided closely with the permanent wilting percentage.
The results in 1936 were similar to those both in 1934 and 1935, and
when plotted, the change in slope of the soil moisture curves occurred
26
University of California — Experiment Station
about June 29 in the first foot ; July 6 in the second, third, fourth, and
fifth foot. Apparently the permanent wilting percentage was not reached
in the sixth foot.
Table 11 gives the average sizes for pears for 1935 and 1936. From this
table it may be seen that while the Bartletts increased in size at substan-
tially the same rate during the early part of the season in 1936 the Winter
TABLE 11
Average Volumes of Pears, University Farm Orchard, Davis, 1935 and 1936
1935
1936
Bartlett
Winter Nelis
Date
Bartlett
Winter Nelis
Date
Wet
Dry
Wet
Dry
Wet
Dry
Wet
Dry
May 18
cc
5.6
9 0
19.9
27.0
35.0
44.6
57.9
71.1
81.6
115.4
134.7
149.2
164.7
179.7
cc
6.7
10 3
15.3
21.3
27.4
34 5
41.1
50.3
60.1
65.5
74.5
81.6
89.0
94.0
100.0
104.3
cc
3.1
4.6
9.4
12.8
16.2
20.2
25.7
31.1
37.2
54.4
65.5
72.8
78.9
85.3
102.2
106.5
cc
3.2
4.8
6.9
9.6
12.8
15.6
19.2
23.2
27.4
31.1
36.6
40.5
46.5
49.7
52.3
55.8
63.1
66.2
May 8
cc
5.7
9.4
13.0
17.5
23.6
30 1
37.7
43 4
63.9
76.2
83.4
92 0
100.1
106.5
117.7
129.7
138.6
cc
6.4
9.4
12.8
17.5
23.3
29 2
36.1
41.6
48.4
55.8
59.4
63.1
66.2
69.5
71.9
76.2
78.9
81.6
cc
4.8
6.5
9.0
12.0
15.0
18.5
22.5
24.4
38.8
45.2
51.6
57.9
61.6
65.5
72.0
78.9
84.3
cc
4.8
May 24
May 15
6.5
May 31
May 22
May 29
9 4
12 3
June 5
June 12 ..
15.9
June 21
20 2
June 28
24.8
July 5
July 12
June 26
July 3
July 10
28.3
33.0
July 19
38.8
July 26
July 17
41.6
August 2
July 24
43.4
July 31
45.2
August 16
August 21
August 29
September 18
September 28
August 7
August 14
August 21
August 28. . .
September 4
46.5
47.1
49.7
51.6
52 3
Nelis in the dry plot grew faster than those in the wet for several weeks
in the early part of the season, as they did in 1934. A marked decrease in
rate of growth of both varieties of pears in the dry plot occurred between
June 28 and July 5 in 1935 and between June 26 and July 3 in 1936.
The pressure tests for the pears from Davis follow :
Variety and ' Pressure test, wet Pressure test, dry
date picked plot, in pounds plot, in pounds
Bartlett :
July 18, 1934 19.8 23.1
July 31, 1934 22.6 24.0
August 5, 1935 28.6 28.9
August 19, 1935 27.1 28.6
July 25, 1936 17.8 21.5
August 8, 1936 17.4 18.1
Winter Nelis :
September 12, 1934 22.7 23.7
September 14, 1936 16.7 19.3
Bul. 667]
Irrigation Experiments with Pears and Apples
27
The pears from the dry plots were consistently harder than those from
the wet. As with the fruit from the other orchards previously reported,
the pressure differences in both varieties disappeared during storage and
in the ripening period thereafter. The keeping qualities of the two lots
were substantially the same.
Prost Orchard. — After a preliminary trial in 1938, two plots of eight
Bartlett pear trees each were used in the Prost orchard near Concord,
Contra Costa County, in 1939. The trees were about fifty years old,
planted on French-pear rootstock 18 feet apart. The summer tempera-
TABLE 12
Moisture Equivalents and Permanent Wilting Percentages
of Zamora Clay Loam Soil from the Prost Orchard, Concord
Depth
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
P.W.P.
feet
Oto 1
per cent
22.6
21.2
20.9
20.9
24.7
per cent
11.3
10.0
10.3
10.5
12.5
ratio
2.00
lto2
2.12
2 to 3
2.03
3 to 4
1.99
4 to 5
1.98
tures are somewhat higher than along the coast, and appreciably lower
than in the interior valleys. The rainfall is variable, but is slightly less
than along the coast. Table 12 shows the water-holding characteristics
of the soil, a Zamora clay loam.
Because rainfall during the winter of 1938-39 was deficient, the wet
plot was irrigated before the first samples were taken, consequently the
wet plot started the season with more moisture present in the fourth and
fifth foot than did the dry plot. Figure 9 gives the soil-moisture record
for 1939. As is shown by the abrupt change in slope of the curve repre-
senting the moisture content of the dry plot on June 7, the permanent
wilting percentage was reached comparatively early in the season. After
June 7 the curves for the moisture contents of the dry plot remained
approximately horizontal until shortly after July 19, when water was
applied to ameliorate the moisture conditions in this plot. The wet plot
was irrigated on June 1 and again on June 22.
Figure 10 shows the average volumes of 100 pears in each plot. The
average increases in size were approximately equal in both plots between
May 10 and May 24, after which the fruit grew more slowly in the dry
plot than in the wet. As the soil-moisture curves show, the permanent
wilting percentage was reached a few days before June 7. After that
28
University of California — Experiment Station
date, the marked differences in average size of the fruit reflect the severe
soil-moisture conditions to which the trees in the dry plot were sub-
jected. The thorough irrigation given to the dry plot shortly after July
19, after a relatively long period when the soil moisture had remained
first foot
SECOND FOOT
20
Id
D
^ A
6"**^ 1
pw.p=io.oa$ "9 6 o o'
THIRD FOOT
Fig.
10 24 7 21 5 19 2
MAY JUNE JULY
. — Soil-moisture conditions in the Prost orchard, Concord, 1939.
The
permanent wilting percentage is indicated by the letters P.W.P. The wet plot
was irrigated June 1 and June 22, and the dry one, July 19.
at the permanent wilting percentage, increased the size of fruit in the
dry plot.
The pressure tests of the fruit picked on July 26 showed 17.7 pounds
for the wet plot, 19.6 pounds for the dry. After the usual period in cold
storage, the fruit was allowed to ripen at a temperature between 65° and
70° F. The differences in hardness tended to disappear as the fruit ma-
tured, and on August 7 both lots were eating-ripe, approximately equal
in quality and firmness.
Bul. 667] Irrigation Experiments with Pears and Apples
29
In preliminary trials in this orchard in 1938, similar results were ob-
tained on the maturity of pears. The soil moisture was reduced to about
the permanent wilting percentage in the dry plot on July 14, 1938, in
the top 5 feet. The pears from the dry plot yielded a pressure test of 23.1
pounds on July 19, whereas those from the irrigated plot gave a test of
21.9. The average test for the entire picking season was 21.1 pounds for
the dry plot, 20.5 for the wet.
130
120
110
to
u 100
90
80
U 70
5 60
D
u 50
? 40
30
20
10
/
7
•
/
/
/
f
^'
10
MAY
24
7 21
JUNE
5
JULY
19
2
AUG
Fig. 10. — Sizes of pears in the Prost orchard, 1939.
OBSERVATIONS ON MATURITY OF PEAR FRUITS UNDER DRY SOIL
CONDITIONS IN OTHER DISTRICTS
Besides the experiments previously described, soil-moisture studies on
the maturity of pears, as measured by the pressure test, were conducted
in several pear-growing districts in the Sacramento Valley and the
Sierra Nevada foothills in 1939 and 1940. The soil samples were taken,
just before and during the picking season, from irrigated and unirri-
gated plots. The soil types ranged from the fine silt of the Sacramento
Delta region to the coarse-textured soils of the Holland and Sierra series,
and to the red clay loam of the Aiken series at about 3,000 feet elevation.
In the orchards where the trees in the dry plots exhausted the readily
available moisture a week or more before picking time, the fruit was 1
to 2% pounds harder than that from the irrigated trees. The data on
the maturity of the fruit from these orchards substantiated those from
the orchards previously described.
30
University of California — Experiment Station
RESULTS WITH APPLES
Hotle Orchard. — The Gravenstein apple trees in the Hotle orchard in
the Gold Ridge district west of Sebastopol were mature productive trees
about twenty years old, planted 30 feet apart. Because the irrigation
water had to be procured from a domestic supply, only two trees were
used in each plot. The soil, classed as a Gold Ridge sandy loam, was gen-
TABLE 13
Moisture Equivalents and Permanent Wilting Percentages
of Gold Eidge Fine Sandy Loam Soil from the
Hotle Orchard, Sebastopol
Depth
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
P.W.P.
feet
Oto 1
per cent
10.0
10.0
10.3
16.2
22.2
20 3
per cent
3.3
3.0
2.9
8.9
10.7
10.2
ratio
3.03
1 to2
3.33
2 to 3
3.55
3 to 4
1.82
4 to 5
2.07
5to6
1.99
erally considered the best apple soil in the district. This orchard was
located in a district of heavy winter rainfall and comparatively cool
conditions during the growing season, with some early morning fogs.
The annual rainfall was about 30 inches.
Experiments were carried out in 1930 by Mr. B. B. Burlingame, a
graduate student, with Gravenstein apples in the Hotle orchard in the
Sebastopol district. Table 13 gives the water-holding properties of the
soil, figure 11 the soil-moisture record for the 1930 season. The orchard
was located on hilly land ; and the soil, apparently, was not uniform over
the area sampled for moisture determinations.
On May 27, when the first set of samples was taken, the soil moisture
in the top 6 feet was at or slightly below the moisture equivalent. Several
heavy rains during April and May were probably the reason why ex-
traction of moisture from the soil had been slow during these months.
Extraction of moisture in the top 3 feet was about the same in the two
plots until July 5. The wet plot was irrigated on July 11, the water
penetrating to a depth of about 6 feet. After July 12, the extraction in
the top 3 feet of the dry plot almost ceased. At all times during the grow-
ing season, apparently, there was an ample supply of readily available
moisture below 3 feet.
Bul. 667]
Irrigation Experiments with Pears and Apples
31
Figure 12 shows the growth of the fruit. The average size of the meas-
ured apples was slightly greater in the dry plot than in the wet at the
beginning of the season and remained so until harvest.
30
20
IO
0
30
20
I Ofe
FIRST FOOT
rrtrrs=7»—— <
'
-
rrrr:
.RW.P = 3.3X
1
SECOND FOOT
DC
= 30
<n
O 20
j 10
O ^
if) o
O 30
IT)
RWP=3.0g
THIRD FOOT
/
/
RWJ?jr_2.9X_
r— <
'-.
) — t
1 !
, — <
( (
20
10
0
K 30
20
10
0
30
20
10
FOURTH
FOOT
[
i ,
r_<
>A
r-<
i
""■'
pwp=8
? t
......
...
FIFTH
FOOT
> <
r— <
r^
LJ
1 >
PWP=I0.7%
i ,
|
SIXTH
FOOT
_H
•- UNIRRIGATED
-i
i
RW.P = l0.2a5
V.
IRR
GAT
ED
1
0
27 14 28 5 12 19 26 2 14
JUNE JULY AUG
Fig. 11. — Soil-moisture conditions in the Hotle orchard, Sebastopol, 1930. The
permanent wilting percentage is indicated by the letters P.W.P. The wet plot was
irrigated July 11.
The readily available moisture in the top 3 feet of soil was exhausted
about the middle of July. The soil moisture contents at this time agreed
closely with the permanent wilting percentages obtained from the
stock supply of soil from the top 2 feet. In the third foot, however,
32
University of California — Experiment Station
the agreement was not close, probably because of the inclusions of the
fine-textured soil in the samples taken for the moisture determinations.
These inclusions were unavoidable because of the undulating surface of
the fine-textured layer in the fourth foot. The fruit in the dry plot began
to grow more slowly than that in the wet one after July 19.
Jerenich Orchard. — The apple trees in the A. N. Jerenich orchard at
Watsonville were of the Yellow Newtown variety, about twenty-five years
old, vigorous, uniform, and exceptionally productive. They were planted
280
260
240
220
200
180
160
140
120
100
80
60
40
y
/
/ y
/ y
/ /
• /
/ y
,'y
sy
s^y
2.1
MAY
14
28
12
JULY
26
JUNE
Tig. 12. — Sizes of apples in the Hotle orchard, 1930.
30 feet apart on the square system. The plots consisted of four trees each,
with the customary guard rows around each plot. The soil was classified
as a Pajaro loam. There was difficulty in interpreting the soil-moisture
conditions, probably because of the gravel present in the top 4 feet and
abrupt changes in texture in the lower depths. The district is character-
ized by fairly heavy rainfall during the winter and mild to cool weather
during much of the growing season. Soil drainage was good to a depth
of 12 feet.
Experiments with Yellow Newtown apples in the Jerenich orchard
were started in 1932, but discontinued after one year because gravel in
the soil made it difficult to obtain a satisfactory soil-moisture record.
Table 14 gives the water-holding properties of this soil. As shown by the
soil-moisture record for the Jerenich orchard in 1932 (fig. 13), however,
the use of water by apple trees is less rapid in the relatively cool coastal
Bul. 667]
Irrigation Experiments with Pears and Apples
33
climate than it is for pears in the warm interior climates. This fact is
important. The slight downward slope of the curves from the dry plot,
particularly in the top 3 feet, indicates very slow extraction of moisture.
The permanent wilting percentage was not reached during the growing
season except in the top foot. Water was applied to the wet plot on June
15 and on August 18, the former penetrating about 5, the latter more
than 6 feet, Both plots had available soil moisture except in the top foot
TABLE 14
Moisture Equivalents and Permanent Wilting Percentages
of Pajaro Loam Soil from the Jerenich
Orchard, Watsonville
Depth
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
P.W.P.
feet
0 to 1
per cent
9.8
7.8
7.8
14.1
14.3
25.3
25.3
13.9
per cent
5.4
4.8
4.8
7.7
7.0
11.5
12.1
7.4
ratio
1.81
1 to 2
1.63
2 to 3
1.63
3 to 4
1.83
4 to 5
2.04
5 to 6
2.20
6 to 9
2.09
9 to 12
1.88
of the dry plot throughout the summer. There were no differences in the
sizes of fruit (fig. 14) that could be attributed to the irrigation treatment.
Two boxes of fruit from each plot for storage tests were secured on
September 5 and on September 20. The pressure tests indicated the same
degree of maturity at picking time of the fruit from the two plots. The
fruit was brought to Davis and stored at 36° F. Pressure tests and ob-
servations on color and quality were made at monthly intervals during
the winter. Early in May, when the fruit was removed from storage, all
showed approximately the same color. There was no evidence of rot;
and the apples were all in excellent condition except for internal brown-
ing, which was about the same in all lots.
Roive Orchard. — In 1933 the experimental work with apples was trans-
ferred to the J. H. Rowe orchard about 5 miles southeast of Watsonville,
where it was continued for two years. The trees in the Rowe orchard were
of the Yellow Newtown variety, about thirty years old at the time of the
experiment, planted 30 feet apart and fairly uniform in size, with a long
record of profitable production. The soil was classified as a Pajaro clay
loam adobe. The top 21/2 feet was typical adobe. Between 2% and 4 feet
was a compact layer of clay containing, apparently, fewer roots than the
15
10
5
0
15
10
5
0
20
15
FIRST FOOT
fcrq
1
^IRRIGATE!
^fc^*
PW.P=5.42
1 1
"V— — 6 -4J 0 — -<>—<>-¥ 6
UNIRRIGATED^
SECOND
FOOT
RWP=4.8^
T T — I — T — Y""T"T 9
THIRD FOOT
30 "
° 25
FIFTH FOOT
£ 20
P
Z 30
Ld
LJ
cl 20
15
30
zs,
PWP=7.02
SIXTH FOOT
r— i
i 1
1 _l
1
^
— i
• — <
c
uj
^^
1 ~"(
1
\—-\
RW.P=II.5*
6 TO 9 FEET
15
30
25
20
15
20
■0 .6
RW.P=I2.I*
9 TO 12 FEET
"--4—4 — y— {—4-4— j— 1-1-1— ~
T 6
F?WP=74j;
28 IF *25 8 22 7 21
MAY JUNE JULY
3 15 24 31 19
AUG SEPT
Fig. 13. — Soil -moisture conditions in the Jerenich orchard, Watsonville, 1932. The
permanent wilting percentage is indicated by the letters P.W.P. The wet plot was
irrigated June 15 and August 18.
Bul. 667] Irrigation Experiments with Pears and Apples
35
layer above it. Below 4 feet the soil was somewhat lighter in texture.
Apparently the clay layer in the second and third feet did not seriously
interfere with drainage. The climatic conditions resembled those de-
280
260
CO
£240
u220
h-200
z
H 180
O 160
CD
D
U
140
Z 120
100
80
60
40.
S
r
31 19
SEPT
Fig. 14. — Sizes of apples in the Jerenich orchard, 1932.
8 21
JUNE
7 21
JULY
4 16
AUG
TABLE 15
Moisture Equivalents and Permanent Wilting Percentages
of Pajaro Clay Loam Soil Adobe from the Eowe
Orchard, Watsonville
Depth
Moisture
equivalent
(M.E.)
Permanent
wilting
percentage
(P.W.P.)
M.E.
P.W.P.
feet
0 to 1
per cent
30.9
31.0
30.8
25.3
20.7
23.7
30.0
29.6
per cent
16.1
16.8
17.1
12.7
8.9
9.7
12.6
11.9
ratio
1 92
1 to 2
1 85
2 to3
1 80
3 to 4
1 99
4 to 5
2 33
5 to 6
2.44
6 to 9
2.38
9 to 12
2.49
scribed for the Nutting and Jerenich orchards. The soil in this orchard
had typical adobe characteristics and held a large amount of water at
the moisture equivalent (table 15). In the top 4 feet, where apparently
most of the roots were growing, about half of the moisture at field ca-
pacity was available for use by the trees.
30
25
20
15
30
25
20
15
30
25
20
u 15
a:
=> 30
g 25
^ 20
=! 15
o
<* 30
O 25
</) 20
Ld
® I 5
z 30
FIRST FOOT
SECOND FOOT
y 6 jf" £
RWPfJ.6.8 X_
Y — ^ — "i — Y — 9 — f
THIRD FOOT
•
"" "*Q """""<
) T
r-— ~*
1 Y
■
RWJ?fl7J_X__
| "|
T"M
FOURTH
FOOT
f"1
i
— r1
i
^^"
■ — -
i
<
P-- 7
i — <
r-T-4— 4— i
PWP=I2.7 X
FIFTH FOOT
, <
,
■ i
>
^
•
V
i
i
f — 1
RW.P*
8.9 X
>
f
\ J
i— 7
5 C
5— I
*-— - C
y ^
SIXTH FOOT
RWP=I2.6*
9 TO 12 FEET
20
15
30
25
20
I 5
RWJ?=M^9_% _
4 18 31 14 28 12 26 9 23 6 20 4
MAY JUNE JULY AUG SEPT
Fig. 15. — Soil-moisture conditions in the Kowe orchard, "Watsonville, 1933. The
permanent wilting percentage is indicated by the letters P.W.P. The wet plot was
irrigated July 19.
1
L..J
p*-*^
r '
> c
£»-<$
F'-"l
*' '
Bul. 667]
Irrigation Experiments with Pears and Apples
37
Figure 15 presents the soil-moisture record for 1933. Water was used
slowly. The permanent wilting percentage was reached only in the
first foot. On June 28 the readily available moisture was exhausted in the
top foot of the dry plot. The curves show slow extraction of moisture to
a depth of about 6 feet. As shown by the records for the 6-to-9- and the
9-to-12-f oot depths, the soil moisture at these levels never got below about
25 per cent, which was but slightly below the moisture equivalent. The
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
,
fSs
4
Y
f
<?/?
^
jr
4 28
JUNE
12 26
JULY
9 23
AUG
6 20
SEPT
4
OCT.
Fig. 16. — Sizes of apples in the Eowe orchard, 1933.
wet plot was irrigated on July 19, the water penetrating a little below
6 feet.
Figure 16 shows the sizes of the measured apples. Fruits from both
plots grew at approximately the same rate throughout the growing
season.
In 1934 the treatments of the plots were reversed, the 1933 dry plot
becoming the wet one. The soil-moisture conditions in 1934 resembled
those of 1933. Table 16 gives the soil-moisture percentages. The perma-
nent wilting percentage was reached in the top foot on August 24 in the
dry plot, but there was readily available moisture at the other depths
throughout the growing season. Since the soil moisture at the 6-to-9- and
9-to-12-foot depths remained at approximately the field capacity all
season, the records for them are not presented.
38
University of California — Experiment Station
TABLE 16
Average Soil-Moisture Percentages, Rowe Orchard, Watsonville, 1934
Date
Wet plot*
0-1
ft.
1-2
ft.
2-3
ft.
3-4
ft.
4-5
ft.
5-6
ft.
Dry plot
0-1
ft.
1-2
ft.
2-3
ft.
3-4
ft.
4-5
ft.
5-6
ft.
April 19...
May 2
May 15
May 30
June 11
June 27
July 11
July 26
August 9 . . .
August 24 . .
September 1
per
cent
22.3
19.0
18.7
17.5
19.3
17.3
24.0
20.4
18.9
18.1
per
cent
26.1
25.1
23.1
21.3
21.6
21.3
26.5
23.2
21.5
20 3
per
cent
23.6
24.4
23.8
22.8
23.1
22.1
25.0
23.7
23.6
22.1
per
cent
18.7
18.2
21.4
20.7
19.2
18.4
26.6
20.7
21.0
19.5
per
cent
15.0
17.4
18.1
14.5
per
cent
20.8
21.5
22.3
20.5
22.8
20.4
17.7
13.9
16.0
15.3
20.4
19.3
20.4
20.3
per
cent
21.5
19.1
18.6
18.7
19.3
17.6
17.6
17.5
15.7
15.3
per
cent
27.1
25.7
24.2
22.7
22.5
21.5
21.0
21.2
20.3
19.9
per
cent
26.5
25.9
24.4
23.6
23.3
22.3
21.8
20.9
20.6
20.9
19.2
per
cent
23.8
23.6
22.4
19.9
20.4
19.2
19.3
19.4
18.1
16.4
14.8
per
cent
24.0
23.7
23.3
21.4
21.3
19.5
17.1
17.7
16.6
15.1
14.9
per
cent
28.3
27.8
26.4
23.4
26.2
25.1
21.4
21.3
19.7
18.8
17.4
Irrigated July 9. 1934.
The volumes of the measured fruits in 1934 were as follows :
Volume, wet Volume, dry
Date plot, in cc plot, in cc
May 2 6.5 9.0
May 15 17.2 23.2
May 30 36.1 48.4
June 12 53.7 71.9
June 27 73.7 104.3
July 11 100.0 132.2
July 26 129.7 172.0
August 9 156.1 203.8
August 23 184.3 239.1
No differences that could be attributed to the irrigation treatment were
obtained. Those in the dry plot were slightly larger than those in the wet
at the beginning of the season, and possibly because of a light crop in
the dry plot, grew faster than those in the wet and were considerably
larger at picking time.
In both years, initial pressure tests were approximately equal.
Storage tests indicated essentially the same keeping qualities for the
fruit from the two treatments. The fruit picked in 1933 kept in good
condition in 36° F storage until May 1, 1934. At that time the propor-
tions of each lot showing internal browning and shriveling were approxi-
mately equal.
J H DISCUSSION OF RESULTS
One fact brought out in these experiments is the comparatively slow
use of soil moisture by mature pear and apple trees in the central-coast
region of California. This condition is shown in the soil-moisture records
Bul. 667] Irrigation Experiments with Pears and Apples 39
for the Nutting, Hotle, Jerenich, and Rowe orchards, where the perma-
nent wilting percentages were not reached except in the first foot and
in the Hotle orchard in the second and third feet. Tiie soils in these
orchards ranged from the fairly coarse soil in the Jerenich orchard to the
fine-textured soils in the Nutting and Rowe orchards. During years of
normal rainfall, when the moisture penetrates to a depth of 6 feet or
more, it is probable that irrigation is unnecessary under these conditions;
and in years of deficient rainfall one or possibly two irrigations to a
depth of 6 feet should suffice.
No responses in growth of the fruit obtained from the orchards in the
coast region seemed to be associated with the irrigation treatment, and,
as a rule, the measured fruits in both treatments grew at about the same
rate throughout the season. Pressure tests from comparable lots showed
no consistent differences, and keeping qualities were not affected. The
rate of growth of the fruits during a given season was not influenced by
the irrigation treatment in those experiments in which there was readily
available moisture in both treatments in all but the surface layer during
the growing season ; yet the irrigated plots had higher soil-moisture con-
tents than the unirrigated plots for considerable periods each year.
The experiments with pears from the University Farm orchard at
Davis, from the Benson orchard in the Lake County pear district, from
the Cleland orchard in Mendocino County, and from the Prost orchard
in Contra Costa County gave results similar to those obtained with
peaches, prunes, grapes, and walnuts. These four orchards are located
in districts where the climatic conditions during the growing season are
characterized by higher temperatures than those in the central-coast
region. The climate at Davis is fairly typical of the interior valley of
California, with a rainless period that generally extends from April or
May until October, and with clear summer days with frequent maximum
temperatures of 100° F or slightly above. In general, the climatic con-
ditions in the orchards in Lake and Mendocino counties resemble those
at Davis except that the winter rainfall is greater and the summer tem-
perature slightly lower than at Davis. The days are generally cloudless
and free from fog during the growing season. In these orchards the soil
moisture usually reached the permanent wilting percentage in the top
4 to 6 feet late in June or early in July. On a few occasions unusual rain-
fall in June served to delay the time when the permanent wilting per-
centage was reached until later in the season.
In the Benson orchard in Lake County, the permanent wilting per-
centage was reached about July 9 in 1935 in the top 4 feet, as indicated
by the flattening of the soil-moisture curves. The exhaustion of readily
available soil moisture was accompanied by a slowing down in the growth
40 University of California — Experiment Station
of the fruit. Fruit growth behaved similarly in 1936 and 1937 when the
readily available soil moisture was exhausted. Clearly, decrease in the
rate of growth if fruit in the Cleland orchard in Mendocino County
occurs about the. time when the permanent wilting percentage is reached
(tables 7, 8, and figs. 5, 6). The readily available soil moisture in the
dry plot in the University Farm orchard was depleted during the first
week in August, and the fruit grew more slowly. In the Prost orchard in
Contra Costa County, the permanent wilting percentage in the dry plot
was reached early in June, because of a deficient winter rainfall, and the
growth of the fruit was retarded at about the same time. Thus, similar
results were secured from both medium-textured (Yolo series) and fine-
textured soils (Dublin and Zamora series) in three widely separated
pear districts.
The maturity of pears as measured by the pressure test was retarded
in the dry plots of the orchards from the interior region, but not in the
coast region. In general, if the available soil moisture was exhausted a
considerable period before harvest, the pressure tests of the pears from
the dry plots were considerably higher than for the fruit from the wet.
On the other hand, if readily available moisture was present until
shortly before picking, little or no difference was found in the pressure
tests of the fruits from the two treatments. These results may help to
explain why pears in certain unirrigated sections ripen comparatively
late, whereas other fruits like apricots, plums, and peaches, which mature
before the soil moisture from the winter rains is exhausted, do not have
their ripening season affected.
As the storage tests revealed, the differences in maturity of pears, be-
cause of the irrigation treatment, tended to disappear in storage and
both lots matured at the same time. When eating-ripe, the pears from
both treatments had approximately the same pressure test ; and various
observers who tried them could detect no marked differences in quality.
The soil-moisture and the growth records indicate that one or two
irrigations before harvest in a normal year, wetting the soil to a depth
of 6 feet, will suffice to maintain readily available moisture until after
harvest in pear orchards in Lake, Mendocino, Contra Costa, and Yolo
counties under conditions like those in the experimental plots. Though
an additional watering may be desirable after harvest, the experiments
were not continued long enough to secure evidence on this question. If
only one or two irrigations are necessary before harvest, conflicts be-
tween irrigation practice and the spraying program may easily be
avoided. The evidence, using the increase in volumes of pears as a cri-
terion, shows that the availability of water did not decrease until the soil
moisture was reduced close to the permanent wilting percentage.
Bul. 667] Irrigation Experiments with Pears and Apples 41
CONCLUSIONS
In the central-coast region in years of normal rainfall, pear and apple
trees on medium- or fine-textured soils similar to those reported in this
bulletin do not exhaust the readily available moisture until late in the
season. Under these conditions, irrigation seems unnecessary.
Mature pear orchards in districts where the climatic conditions some-
what resemble those of the interior valley, such as Lake, Mendocino, and
Contra Costa counties, exhaust the soil moisture to the permanent wilt-
ing percentage in the top 4 to 6 feet of soil about the last week in June
or the first week in July during normal years. Under these conditions,
the trees, if growing on soil at least 6 feet deep, may be kept supplied
with readily available water with one or two irrigations before harvest.
When the readily available soil moisture is exhausted, the pears slow
down in growth on both medium- and fine-textured soils. But when the
permanent wilting percentage is not reached while the fruit is on the
trees, the growth of the fruit is not retarded.
The apple experiments were conducted under mild climatic conditions.
Under these conditions when the readily available soil moisture below
the first foot is exhausted, the apples slow down in growth, but when the
permanent wilting percentage is not reached, the growth of the fruit
is not retarded.
The maturity of pears (as measured by the pressure test) is delayed
if the readily available soil moisture is exhausted a week or more before
the normal harvest period. Differences in maturity between fruit from
the irrigated and from the unirrigated plots tend to disappear in storage.
According to these experiments, pears grow normally under a wide
range of soil and climatic conditions when the trees are kept supplied
with readily available soil moisture, but deleterious responses result
when the soil moisture is reduced to about the permanent wilting per-
centage during the growing season. These responses are generally shown
by decreased size and delayed maturity.
42 University of California — Experiment Station
LITERATURE CITED
1. Aldrich, W. W.
1937. Six years' study of pear irrigation. Oregon State Hort. Soc. Proc. 29:
79-87.
2. Aldrich, W. W., and Arch Work.
1933. Preliminary report of pear tree responses to variations in available soil
moisture in clay adobe soil. Amer. Soc. Hort. Sci. Proc. 29:181-87.
3. Aldrich, W. W., and R. A. Work.
1934. Effect of leaf -fruit ratio and available soil moisture in heavy clay soil
upon amount of bloom of pear trees. Amer. Soc. Hort. Sci. Proc. 31:57-74.
4. Aldrich, W. W., and R. A. Work.
1935. Evaporating power of the air and top-root ratio in relation to pear fruit
enlargement. Amer. Soc. Hort. Sci. Proc. 32:115-23.
5. Aldrich, W. W., R. A. Work, and M. R. Lewis.
1935. Pear root concentration in relation to soil moisture extraction in heavy
clay soil. Jour. Agr. Res. 50:975-88.
6. Boynton, Damon.
1937. Soil moisture and fruit growth in an orchard situated on shallow soil in
the Hudson Valley, N. Y. Amer. Soc. Hort. Sci. Proc. 34:169-72.
7. Furr, J. R., AND E. S. Degman.
1932. Relation of moisture supply to stomatal behavior of the apple. Amer. Soc.
Hort. Sci. Proc. 28:547-51.
8. Furr, J. R., and J. R. Magness.
1931. Preliminary report on relation of soil moisture to stomatal activity and
fruit growth of apples. Amer. Soc. Hort. Sci. Proc. 27:212-18.
9. Haller, M. H., and P. L. Harding.
1938. Relation of soil moisture to firmness and storage quality of apples. Amer.
Soc. Hort. Sci. Proc. 35:205-11.
10. Hendrickson, A. H., and F. J. Veihmeyer.
1929. Irrigation experiments with peaches in California. California Agr. Exp.
Sta. Bui. 479:1-63.
11. Hendrickson, A. H., and F. J. Veihmeyer.
1934. Irrigation experiments with prunes. California Agr. Exp. Sta. Bui. 573:
1-44.
12. Hendrickson, A. H., and F. J. Veihmeyer.
1937. The irrigation of pears on a clay adobe soil. Amer. Soc. Hort. Sci. Proc.
34:224-26.
13. Lewis, M. R., R. A. Work, and W. W. Aldrich.
1934. Studies of the irrigation of pear orchards on heavy soil near Medford,
Oregon. U. S. Dept. Agr. Tech. Bui. 432:1-33.
14. Lewis, M. R., R. A. Work, and W. W. Aldrich.
1935. Influence of different quantities of moisture in a heavy soil on rate of
growth of pears. Plant Physiol. 10:309-23.
15. Magness, J. R., E. S. Degman, and J. R. Furr.
1935. Soil moisture and irrigation investigations in eastern apple orchards.
U. S. Dept. Agr. Tech. Bui. 491:1-36.
Bul. 667] Irrigation Experiments with Pears and Apples 43
16. Ryall, A. L., and W. W. Aldrich.
1938. The effects of water supply to the tree upon water content, pressure test,
and quality of Bartlett pears. Amer. Soc. Hort. Sci. Proc. 35:283-88.
17. Ryall, A. L., and F. C. Reimer.
1937. The effects of water supply to the tree upon storage and dessert quality
of Anjou pears grown in clay adobe soil in the Rogue River Valley. Ore-
gon State Hort. Soc. Proc. 29:74-78.
18. Veihmeyer, F. J., AND A. H. Hendrickson.
1936. Essentials of irrigation and cultivation of orchards. California Agr. Ext.
Cir. 50:1-24.
19. Veihmeyer, F. J., and A. H. Hendrickson.
1938. Soil moisture as an indication of root distribution in deciduous orchards.
Plant Physiol. 13:169-77.
20. Work, R. A.
1937. The control of soil moisture. Oregon State Hort. Soc. Proc. 29 : 41-46.
21. Work, R. A., and M. R. Lewis.
1936. The relation of soil moisture to pear tree wilting in a heavy clay soil.
Amer. Soc. Agron. Jour. 28:124-34.
12m-4,'42(8700)
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